In the field of thin film transistor liquid crystal display, usually, a trolley is required to transport substrates or the like from one position to another. To make the position of the transportation accurate, one or more identification codes recording positional information are provided on the track along which the trolley moves. On the trolley is provided an identification code reader. The positioning of the trolley may be achieved by reading the positional information recorded in the identification code using the identification code reader. If the position of the identification code reader is shifted due to any external force or the like, the positional information read by the identification code reader may be inaccurate, resulting in an inaccurate positioning of the trolley. Thus, the position of the identification code reader itself can affect the accuracy of the positioning of the trolley.